As a conventional process and device for monolithic molding of a synthetic resin pipe, it is known to use a mold which comprises a metal floating core having a diameter corresponding to an inner diameter of the pipe and an injection port for injecting a compressed fluid, both provided at one end of a main cavity, and a sub-cavity provided at the other end of the main cavity through a communication port; the main cavity is filled with a molten resin, the compressed fluid is introduced under pressure through the injection port, the floating core is moved toward the communication port to form a hollow part in the resin filled within the cavity, and an excess part of the resin is discharged into the sub-cavity through the communication port, thereby monolithically molding a hollow pipe (Japanese Patent Laid-Open Publication No. Hei 4-208425).
The above-described conventional process and device are primarily for injection molding and are particularly characterized by forming the hollow part by effecting the injection of the compressed fluid and the movement of the floating core, and have an advantage of producing a pipe which has a smooth surface condition and high dimensional precision. And, they can also mold a reinforced resin containing reinforced fiber such as glass fiber, carbon fiber or metal fiber. Besides, they have an advantage of producing a curved pipe in addition to a straight pipe when bending is within a range that the floating core can be moved.
And, in the above conventional process and device, the communication port has a diameter smaller than the floating core, and the floating core stays within the main cavity. Thus, since the metallic floating core is designed to stay within the main cavity, it is advantageous that the resin extruded into the sub-cavity can be pulverized after completing the molding process and can immediately be used again as the molding material.
However, the above conventional process and device had the following disadvantages.
(1) Since the floating core remains within the main cavity, a molded article has at least one end of its formed hollow part closed by this floating core. Although this hollow shaped molding process and its molding device are particularly suitable to produce a pipe, the produced pipe has its one end closed by the floating core.
(2) The above closed part is cut off after molding. But, it is a troublesome work, and the metallic floating core remains in the cut-off piece. Therefore, the floating core must be removed before reusing the cut-off piece as the molding material.
(3) The resin discharged into the sub-cavity is removed as a block- or rod-shaped mass and cannot be pulverized easily, detrimental to reuse. Particularly, when a hollow shaped molded article is made of a reinforced resin containing reinforced fiber, the reinforced resin in the sub-cavity has a high strength, so that a large crusher is required to break it, and its blades are easily worn. Thus, the reinforced resin cannot be reused readily. Besides, reuse is also hindered because when the reinforced resin is crushed, its reinforced fiber is cut into pieces, lowering the mechanical properties of the reinforced resin.
(4) When the hollow shaped molded article is made of a reinforced resin, the inner surface of the hollow shaped molded article with which the moving floating core is in direct contact loses its smoothness easily because the reinforced resin contains a filler reinforcing material or reinforcing fiber. Therefore, when the hollow shaped molded article is used as a pipe, for example, there is a disadvantage that a significant property of the pipe, i.e., a flow resistance, is increased, resulting in extremely limiting its application range.
(5) To move the floating core, the compressed fluid at a substantially high pressure is injected in a stroke. Therefore, the floating core is instantaneously moved toward the communication port. As a result, the floating core does not always pass through the center of the main cavity and often passes a deviated route, tending to increase a deviation in wall thickness of the hollow shaped molded article. Particularly, to produce a curved pipe, there is a high tendency for the floating core passes by the inner radius of the curved part, causing a prominent deviation in wall thickness. On the other hand, since the floating core is hardly moved when a compressed fluid is injected slowly under a low pressure, it is quite difficult to control the moving speed of the floating core by the pressure and injecting speed of the compressed fluid.
The present invention has been completed in view of the above conventional disadvantages and aims to achieve the following objects.
Specifically, a first object is to make it possible to produce a molded article with either end of a hollow part open without taking a labor of cutting a closed part after molding.
A second object is to produce a hollow shaped molded article having an inner surface with particularly good smoothness when a reinforced resin is used to produce it.
A third object is to produce a hollow shaped molded article having a quite uniformly distributed wall thickness.
And, a fourth object is to further facilitate the reuse of an excess resin discharged from the main cavity.
To achieve the above objects, means provided by the invention are classified roughly into three.
First, description will be made on a hollow shaped molding process and device according to a first aspect of the invention to achieve the first object.